What makes the AR-15 so accurate?

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kotengu

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I've heard (and seen, for that matter) that the AR-15 is the most accurate gas-operated semi-automatic rifle many times now, and as a mechanical engineering student I wonder a lot about the "whys" behind facts a lot, so........

What is it, exactly, that sets the AR apart from the others? Assume we're comparing match-grade, suped up versions (with tailored handloads, match barrels, and great triggers) of each so we'll compare apples to apples (sort of - I guess we could add in ANY semi-auto rifle as well). I have four basic ideas (more like wild guesses) that I'd like people to comment on, and add to them if possible:

1. Direct gas impengement: Fewer heavy reciprocating parts leads to less variation from shot to shot.

2. Longer "dwell time": (guessing here - anyone with empircal data on this would be appreciated) The distance the expanding gases have to travel before they begin motion of anything (bolt carrier) leads to the bullet being long gone before ANY objects start to move to cycle the action, lessening the chance for shot-to-shot variation.

3. Rotating bolt: Is the locking lug design a contributor? Why is it better than other rotating bolts (ie: M14/M1A), tilting bolts (ie: FAL), or the HK system?

4. Simply the caliber: I would count the lack of recoil as making the rifle easier to shoot, but not inherantly more accurate. Is the round itself (.223) inherantly more accurate than say, a .308?

I'm mostly looking for the "whys" behind accuracy, so please let's not turn this into a "mine's better than yours" thread - just a good, in depth discussion of main contributors to accuracy.
 
It takes a great deal of smithy work for an AR to be really accurate. Inherently, not "inherantly", the AR is ok, but not a run of the mill, right out of the box, rifle. That's just a rifle that is ok for just shooting, but if you're talking a Camp Perry rifle, that's totally different. That's a custom barrel with a custom trigger and tuned for match grade ammo.
Accuracy is a combination of a whole bunch of things. A good barrel, good trigger, good sights, good ammo all contribute to an accurate rifle. Your "suped up" rifle has had all this done to it to shoot, usually a specific load, for a specific purpose, as well as possible.
If you can find a copy, there's a book called The Accurate Rifle. Jack O'Connor, I think, that will give you lots of good info. Try the library.
 
From what I've heard, the BAR was extremely accurate- perhaps too accurate for a fire support role.
 
For the most part I believe the cartridge is responsible for much of the accuracy. No violent recoil, good muzzle velocity and stable bullet profile accounts for a lot.

Chamber the AR in something else...say the 9mm and you won't get near the same accuracy.

I wouldn't think that the action type would have much of an effect on accuracy considering the bullet should have left the barrel prior to the action actually working. This is of course taking in consideration that bolt lock up is consistent.

Good Shooting
Red
 
Its not the caliber becasue the mini 14 is also a 223 and it shoots like crap. Its more to do with the direct gas system. The AR10 is typically more accurate than other 308's as well.
Pat
 
IMO, part of the AR's accuracy is the gas system. And I don't believe that it is just tuned match rifles that are incredibly accurate. My dad has an old Colt SP-1 that is more accurate than all but two or three of the other rifles in the house. It is NOT more accurate than my Armalite NM or my brother's Walther KK100. With ball ammo it will outshoot the M-1 Garand and 1903A3; with tuned ammo the 03A3 has the edge, barely. Just my observations, so take them as you will.
 
I think its probably the fairy dust they sprinkle on when it leaves the factory. Green Label is the best because the fairies that year used industrial grade dust and not that cheap glitter imported from PRK. :neener:
 
I think a weapons ability to stabilize the bullet has a lot to do with accuracy, so the load and the rifle would equally important in this regard. That being said, it may simply be that the AR-15 / 5.56 combination is particularly accurate because the load and the weapon have been well matched. I have also heard that the shorter and fatter the cartridge, the faster and more evenly the powder burns, thus making a more consistent and accurate round. I don't know if this is true, but it could have something to do with it.
 
Its not the caliber becasue the mini 14 is also a 223 and it shoots like crap.
Click to expand...

I would think that you could chamber the Mini 14 in any caliber and it would shoot like crap! :D

Actually there are tuned bull barelled versions of the Mini 14s out there that will match the AR in accuracy.

Good Shooting
Red
 
"It takes a great deal of smithy work for an AR to be really accurate."

Nothing personal, but this is not true. A box stock AR15 is a very accurate rifle. VERY accurate. The difference between it and a Camp Perry AR15 is very little. Unlike pretty much any other "Match Grade" rifle, the AR15 takes very little to make it capable of winning it all at Camp Perry.



*Not that I have ever shot at Camp Perry, but I have read a number of books about what it takes to make an accurate AR15 including (but not limited to) The Complete Guide To AR-15 Accuracy by Derrick Martin & Barrett Tillman, and The Competitive AR15 by Glen D. Zediker
 
444,
I've personally come to believe that with an AR it is the quality of the bullets that makes all of the difference in the world. (Just a roundabout way of saying, I agree with you.)

A friend of mine, who is a retired NYPD detective, had/has a hard time believing that an AR will outshoot an M1A/M14/M-1 Garand. So I took him shooting. Now he's a believer.
 
Hey Red...

You're talking about buying an aftermarket bull barrel, right? I think you're right about that improving accuracy. Have you seen a Mini-14 with a bull barrel? Does it look OK? Are the bull barrels stainless or chrome lined? I've not given up on Minis yet. (I wonder how much it costs?)
In the opinion of the guys on the board~~All the bitching about accuracy we do (and others do), why doesn't Ruger address this problem?

Thanks,
KR
 
From what I've heard, the BAR was extremely accurate- perhaps too accurate for a fire support role.
Click to expand...

The Army included the BAR in their tests before selecting the Garand. The BAR was decently accurate and was made famous byt the "wing shooting" demonstration done when it was first fielded.

Here's an apples to oranges test in that the BAR had about a 50% higher rate of fire, But in the Infantry Board tests of 1931, the BAR had an average value on each target hit of 3.50, while the Garand scored 3.96 and the Springfield M1903 4.09.

The board noted that the practical differences were minimal. Don't forget, though, that service BARs would heat up, making them less accurate, and shoot out the barrels more rapidly because of their higher rate of fire. An impressive weapon even today.
 
Thanks for all the replies -

Don't get me wrong, I HATE the mousegun. I hate the fact that it "poops where it eats", that those silly little locking lugs and recesses are such a pain to clean, that it goes "sproing" when you shoot it, and most of all, the terminal ballistics of the .223 at any range of significance.

However, it seems that we can't have the good without the bad. I guess what I'm trying to figure out is if there's a way to isolate or reproduce what the rifle does RIGHT, and not have all the things that it does WRONG. It's looking like they are mutually exclusive, but for the sake of argument - let's play devil's advocate to some of the ideas posted above:

1. Straight line motion/design, with low bore axis: The FAL has a very straight line design, with the rat tail being the only deviation. The gas piston does not have much mass, but does sit slightly above the bore so I suppose it could impart a moment on the gun as it cycles. The bore axis of the FAL is low as well. I can see how the M14 sets up quite a torque/moment as it cycles, with that crazy op-rod deisgn. Is it then the bolt lock up that keeps the FAL from being as accurate?

2. Low mass of moving parts: What is the mass of the bolt/carrier combo in the AR? I'd be curious to see how some actual measurements of the weight of moving parts compare in different guns. Intuitively I would guess that the AR's are the lightest, followed by the FAL (although it does have a pretty massive carrier), then the M14, with that huge op-rod assembly, and I'm afraid I don't know much about the HK. If anybody can post some measurements of what they've got, I'd appreciate it. I'm still not convinced that it's the WEIGHT of the moving parts that's significant, and not WHEN those parts move. I don't suppose anyone has a high-speed camera we can take some video of all these guns in action with, do they? I've tried modelling the gas expansion for the FAL to see mathematically when (and how) these actions occur, but it's just too complicated with all the ports, vents, and sharp corners, not to mention the gasses moving (and expanding) at greater than the speed of sound, which complicates things more.

3. Bolt lock-up: I can see how the locking design of the AR lends itself to a tight lock-up, but then how is it that the M14 rotating bolt is not as strong/tight? Or is it that it IS just as good, but the crazy op-rod design overshadows it? I know the AR uses more (seven?) lugs and locks into a barrel extension, as opposed to the M14 that uses two (?) lugs and locks into the receiver, but does it make that much of a difference? Is the surface area of the actual mating parts that much greater (or maybe better distributed) on the AR vs. the M14?
 
Excellent questions.

On the subject of bolt lock-up. The AR-15 uses a multi-lug rotary bolt that locks into the barrel extension. This has a couple of advantages over the tilting block and rotary lug/reciever systems. The multiple lugs lock the bolt radialy around its circumference, which makes for better repeatability and tighter engagement. The multiple lugs also allow the bolt to unlock with less overall movement.

The M14 system uses a similar system, but with only two lugs which lock into slots in the reciever. This is not as good a system, as it causes any play between the barrel and reviever to affect the lockup. Also, two lugs will not lock as closely as seven.

The FAL system is easily the worst. The tilting bolt is effectively a single lug, and locks onto the locking shoulder at the rear of the reciever instead of the front. Not nearly as consistent as a rotating bolt, and much more vulnerable to wear.

- Chris
 
444 if very correct that a Camp Perry rifle (AR anyway) is not very different from a regular rifle off the shelf. TheAR is what it is because it is built more like an Olympic free rifle than a hunting rifle or what we have known to be "battle rifles" of the past. The design is modular, the gas system does not affect the barrel position or harmonics, there is no wood to distort the gun on a day to day basis, and the barrel is essentially mounted at one spot only (two for non-floated guns). The guns are acutally easy to build and easy to make accurate. All you need is a decent barrel. The modularity prevents you from having to tune the rifle, you just screw in a barrel and it shoots. All the stuff that Glenn Zediker and countless others write about is how to get the ultimate from it, and that is all. You can get 95% out of an AR without any of that stuff. It is mainly for competitors that need to have a rifle perform a specific way, without doubt, for it's lifetime. The avereage plinker would not notice the differences.
 
Gas impingment action on fewer moving parts. No op rod, op rod guide or op rod guide spring to ruin barrel harmonics. Also, the buffer & buffer spring is in the stock & not contacting the barrel. Fewer stuff the better.
 
Can't forget the factor of the fifteen pounds of stuff every "tactical" AR owner tends to slap on.
 
I don't know much about guns, I'm a pull trigger go bang guy. But I don't think so much that it's accurate, but it's so darned fun and easy to shoot.

For us not mechanically inclined, userfriendliness goes a long way.
 
Kentucky,

I've only seen one in real life. That belonged to one of the guys that I worked with. They seemed to be pretty nice, except I don't think his was a Clark Custom gun.

Did a bit of searching and found Clark Custom. They want $795.00 to convert your Mini 14 to a bull barrelled rifle.

http://www.clarkcustomguns.com/m14hbar.htm

clark93.jpg


Function and accuracy test (1.5" at100 yards).

So yes, you could turn your Mini 14 into a potential 1" gun with the right ammo. But when you figure in $795.00 plus $400 for the base gun you are looking at almost $1200. That's a good bit more than a nice Bushy or Armalite AR. About even with a NM type AR.

Now Accuracy Systems has a much more economical approach.

http://accuracysystemsinc.com/ruger_mini_prices.html

Ultra Match 1" MOA OR BETTER = $490 Stainless/ $450 blue
Standard Match 1.5" MOA OR BETTER= $450 Stainless/ $399 blue

Figure 400 for the rifle and $450 for the blue Ultra Match and you can have a sub 1" gun for the same price as an AR.

k001b.jpg



I'm tempted myself to get one done just so I can say I have a Mini 14 that will beat your AR! :D

Good Shooting
Red
 
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